Portal Configuration

Should implement the loading of the portal configuration. The portal manager should be implemented as a service, which offers the data configured in the portal xml files.

- [x] implement the entity classes completely
- [x] finish portal manager (service)
- [x] implement tests
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no
| Fixed tickets | no
| Doc           |  https://github.com/sulu-cmf/docs/blob/master/developer-documentation/300-portals/portal-component.md https://github.com/sulu-cmf/docs/blob/master/detail-specification/300-portal/DET-302-portals.md